What are veins?

Veins are blood vessels of different sizes that transport deoxygenated blood from every part of the body back to the heart. They are part of the circulatory system, also known as the vascular system.

All blood vessels in the body, the arteries, veins and capillaries, have the same basic structure. Each have three layers in their walls:

Intima – The innermost layer

Media – the middle layer that is variable thickness and made up of smooth-muscle cells

Adventitia – the outer layer that supplies nutrients to the living cells of the vessel wall

The innermost surface of the intima, i.e. the surface that is in contact with the blood, is covered with a single layer of flat cells called the endothelium (the individual cells being endothelial cells). These are very specialised cells that allow the blood flow smoothly over them, without it sticking to them or starting to form clots.

It is the job of the intima to support the endothelial cells and to provide a structure for the endothelial cells to hold onto and to obtain nutrition from.

The intima and adventitia are very similar in all blood vessels – the difference between the vessels is mainly the thickness of the media layer.

If you compare the diagrams of the structure of the arterial wall and the venous wall, you will see the relative thicknesses of the layers and how the three layers are orientated. In medical textbooks we always colour the arteries red and the veins blue. However in real life, empty and healthy arteries and veins are bright white.

It is the colour of the blood within the arteries and veins that give them their traditional colours. Most arteries in the body carry oxygenated blood which is bright red and most veins carry deoxygenated blood which is darker and more blue.

The circulation is split into two parts – the circulation of the body, legs, arms and head which is called the “systemic” circulation and the circulation of the lungs which is called the “pulmonary” circulation.